Method and apparatus for obtaining pressure build-up data in pumping wells



NOV. 18, 1969 M K, STRUBHAR ET AL 3,478,584

METHOD AND APPARATUS FOR OBTAINING PRESSURE BUILD-UP DATA IN PUMPING WELLS Filed Deo. 26, 1967 "dooooo" ooooo MALCOLM K. sTRuBHAR vAsEL'R. sLovER,JR.

United States Patent 4Of U.s. Cl. 73 1ss s lClaims ABSTRACT OF THE DISCLOSURE This specification discloses a pumping well equipped to obtain pressure build-up data. The well is provided with a packer which separates the production interval of the well from the inlet of production tubing equipped with a downhole pumping system. A passage extends from below the packer to the well annulus above the packer. A remotely controlled valve is located in the passage and a pressure sensing device is located within the well below the packer. In order to obtain pressure build-up data the well may be pumped at a constant rate and the valve thereafter closed to shut in the well at the face of the formation. The pressure below the packer can then be measured to obtain a pressure build-up curve.

Background of the invention This invention relates to a method and apparatus for obtaining pressure build-up data in wells and more partic ularly to a method of obtaining such data in "pumping wells.

It is well known to test a well which produces from a subterranean formation in order to gain information regarding the well and the formation from whichthe well is producing. For example, in the petroleum industry, oil wells periodically are placed on test in order to gain information on the pressures within such wells under various conditions. As is well known in the art, the rate of production of a well varies with the differential between the pressure within the formation penetrated by the well and the pressure within the well opposite the forma tion. This latter pressure is commonly termed the bottomhole pressure of the Well. The bottomhole pressure of a well at any given production rate is designated as the "operating pressure of the well for this production rate. If the formation pressure and the operating pressure for a given production rate are known, it is possible of course to calculate the pressure differential necessary to maintain this rate.

The pressure of a formation in the portion thereof adjacent a well may be determined by a so-called pres sure -build-up test in which the well is shut in for such time as is necessary to establish a pressure equilibrium be= tween the well and the formation. During this shut-in period the bottomhole pressure of the well is measured so that a pressure build-up curve, i.e., a plot of pressure versus time, may be obtained. The pressure measurements may be obtained by means of a pressure measuring and recording instrument, commonly termed a pressure bomb, which is positioned Yat a suitable location lwithin the well. The operating pressure of the well may lilten wise be determined with such pressure recording devices while thel well is on production'. Thus, while the well is produced at a certain rate, a pressure bomb may ne positioned within the well adjacent the formation and the de- ICC sired measurements taken. Thewell during such test may be produced at two or more different rates in order to obtain a correlation of operating pressure and production rate.

Bottomhole pressure measurements of a well taken either as a function of production rate or time or both give valuable information regarding various factors such as the permeability of the formation and possible damage to the face of the formation at the wellbore. Numerous analytical techniques are known for determining various well and/ or formation characteristics on the basis of such pressure-rate-time data. For example, in a paper by D. R. Horner, Pressure Build-Up In Wells, Proceedings Third World Petroleum Congress, Section II, Drilling and Prou duction, The Hague, 1951, page 503, there is disclosed an analytical technique for determining formation conductivity and reservoir pressure.

- One procedure for obtaining pressure build-up data for pumping wells is the so-called lill-up technique. This technique involves stopping the pump and allowing oil to accumulate in the well until the equilibrium head necesu sary to offset the reservoir pressure is reached. The Ibot tomhole pressure may be determined through use of a pressure bomb .located near the bottom of the well. The pressure bomb may be installed on the production tubing or it may be suspended in the annulus between the tubfing and casing or in a separate parallel string of tubing by suitable wire line techniques. Alternatively, the level of the liquid within the well may be noted at suitable intervals of time as it rises in the annular space surround ing the production tubing. The hydrostatic pressure gradient of the liquid column within the well then can be estimated in order to calculate bottomhole pressure values.

The above-described till-up technique is subject to serious limitations and disadvantages. For example, where the bottomhole pressure is measured indirectly by mon-1 itoring the liquid level in the annulus, aserious potential error resides in the estimation of the hydraulic gradient of the column of uid in the annulaus. A value for the gravity of the oil at the surface, of course, can be de= termined from a sample of the oil obtained at the wellhead. However, this offers only an approximation of the gravity of the oil as it exists within the well. In this regard, the oil as it enters the well from the formation may have entrained and dissolved therein an unknown and sub= stantial amount of gas which will reduce the specific gravity thereof by an unknown amount. Also, the well may produce-varying amounts of water which, although it affects the'hydraulic gradient of the liquids within the `well to a lesser extent than gas, also may introduce an error.

This source of error is not present in a fill-up test in which the bottomhole pressure is measured directly by means of a pressure bomb suitably` located in tlie well. However, another disadvantage found in thell-iip test regardless of the mode of pressure L'niea'su'rerhent used, is that although the well nominally is shut-in during the test, the well still is producing relative to the formation in the sense that oil is flowing from the formation into the well. Thus, a truly shut-in condition of the well with respect to the formation is not achieved until the equilibrium head of uid within the well necessary to offset the formation pressure is reached.

One technique for obtaining pressure build-up data without the disadvantages noted above is disclosed in U.S. patent application Set. No. 555,465 by V. R. Slover, Jr., et al., filed lune 6, 1.966 and now US. 'Patent 3 3,410,137. In this procedure, the production tubing is closed to the inflow of liquid through the inlet thereof and gas is injected into the well annulus preferably at a rate such that the liquid level in the annulus is maintained substantially constant. The liquid level and the gas pressure in the annulus are monitored in order to determine the bottomhole pressure of the well during the test.

Summary of the invention In accordance with the present invention there is provided a new and improved method for obtaining pressure build-up data in a well. The invention is practiced in a well provided with a packer located above the open production interval of the well, In carrying out this method, liquid is produced from the well by ilowing the liquid through a passage extending upwardly through the packer into the well annulus above the packer and withdrawing it from the annulus to the surface through a tubing withinl the well. The pressure in the well below the packer is sensed. Thereafter the passage is closed to cause a pressure build-up Within that portion of the well below the packer and the pressure within this zone of the well is sensed.,

In a further aspect of the invention, there is provided a new and improved well system for use in obtaining pressure build-up data. The well is provided with production tubing having an inlet therein which provides for :duid communication between the interior of the tubn ing and the well annulus exteriorly of the tubing, The tubing is provided with a pumping system which func tions to withdraw liquid from the annulus and displace it upwardly through the tubing to the surface of the well. Packing means within the well separates the tubing inlet and the open production interval in the well. The system further includes means forming a passage which extends through the packing means and provides for fluid communication between the open production interval and the aforementioned annulus above the packing means. A valve is located in the passage in order that it may be closed to uid ow and a pressure sensing device is 1ocated in the well below the valve.

Brief description of the drawing The drawing is an illustration, partly in section, showu ing a, preferred embodiment of the inventione Description of the preferred embodiments With reference to the drawing, there is shown therein a wellbore which extends from the surface of the earth and penetrates a subterranean oil producing forrna= tion 12. The wellbore may be provided with` a casing string 14 which is set to the top of the producing forma.n tion 12 and cemented as indicated by reference numeral 16, The uncased portion of the well indicated by bracket 18 defines an open production interval within the Well through which uids may enter the well from the sur rounding subterranean formation 12, Although, in the well illustrated, the open production interval is defined by an uncased section of the well, it will be understood that other suitable completion arrangements may be used For example, rather than the open hole completion shown, the well may be cased and cemented to its total depth and the casing and surrounding cement sheath then guns or jet-perforated opposite the formation 12 in order to define an open production interval for the Well. Such procedures of providing -an opening in a well to the low of subterranean uids are well known in the art and therefore will not be described further.

A production tubing string 20 is disposed within the Well and extends from the surface to a suitable depth within the well. Liquid from the substerranean formau tion 12 accumulates in the bottom of the well and is produced through the interior of the tubing string 20 to the surface by means of a suitable bottomhole pumping system which, as shown, may take the form of a. con-- 4 ventional sucker-rod pumping system. This system comq prises a standing valve assembly 22 disposed within the tubing 20, a traveling valve and plunger assembly 24, and a sucker rod string 26 for movement of assembly 24. The sucker rod string 26 extends through a stufling box (not shown) at the top of tubing string 20 and is connected to a suitable prime mover which functions to reciprocate string 26 and assembly 24. The pumping sys tem illustrated is well known to those skilled in the art and accordingly will not be described further,

In accordance with 'the present invention, the well is provided with a stinger assembly connected to the botm tom of tubing string 20. This assembly comprises a conI duit 27 which extends through a packer 32 and supports a pressure sensing device 28 .in the ybottom of the well. Located in conduit 27 is a perforated nipple 30 which functions as an inlet for fluid accumulated in the bottom of the well and a valve 34. Preferably the stinger assembly is connected to the tubing string 20 by means of a downhole separator 36, which in addition to reducing the gas content of the liquid handled by the pumping system, provides for a standing column of liquid in the annulus 37 above the packer 32 for a reason which will appear hereinafter. The separator 36 is provided with a riser pipe 38 and one or more ports 40 which provide an inlet to the bottomhole pump.

The valve 34 may be of any suitable type which can be remotely controlled from the surface of the well.` In tests caried out in respect to the invention, the valve used was a two-way valve normally held open by a spring and adapted to be closed by application of pneumatic pressure to a side control porta Such pressure was ap plied by means of a small control tubing (not shown) extending from the surface of the well, The pressure gauge 28 preferably is of the surface-indicating type which includes means for intermittently or continuously producing a signal representative of the sensed pressure for application to suitable indicating and recording means at the surfacea Communication from means 28 to the surface may be provided by means of an electrical conductor (not shown) which can be clamped, along with the control tubing, to the tubing string 20,

-During normal operation, the well is produced as folc lows. Fluid from formation 12 flows into the well and thence into conduit 27 through the inlet provided by perforated nipple 30. The uid flows upwardly through the packer 32, valve 34, and riser pipe 38 into the annulus 37., Such gas as is separated from the produced liquid passes upwardly through the annulus to the surface of the well and the liquid accumulates in the annulus 37 above packer 32. The liquid enters 'the tubing string :20 through inlet ports 40 and is withdrawn to the surface of the Well by action of the sucker rod pumping systeme In making a pressure build-up test in a well in accordu ance with the instant invention, itis preferred to establish first the operating bottomhole pressure of the well for a given pumping rate. This is accomplished in the preferred embodiment of the invention by producing the Twell at a constant rate which is less than that required to cause pumping 0E, i.e., lowering of the liquid level in the annulus 37 to a. point below the inlet ports 40n As the well is pumped at the desired rate, it ultimately will reach an equilibrium condition at which the flow rate from the formation into the well will substantially equal the flow .rate from the well through the production tubing 20 At this equilibrium condition, the liquid level within the annulus 37 above packer 32'wi1l be substantially constant with only a small amount of fluctuation due to the action of the pump as it intermittently withdraws uid from the annulus into the iioW tubing 20 on each upstroke of the plunger and traveling valve assembly 24.

In testing a well having fairly well delined and con-1 stant production characteristics, a suitable pumping rate leading to the above-described equilibrium condition can usually be established without great diiculty, particularly with the aid of pressure readings taken by means 28. However, in some cases it will be desirable to directly monitor the liquid level in annulus 37 as the pumping rate is adjustedv to the desired constant rateifThis may be accomplished by any suitable well sounding technique. For example,'a liquid-level detector of the sonic type such as that disclosed in U.S. Patent No. 2,232,476 to Oo F. Ritzman may be used.,

The constant production rate of the well should be maintained for an edequate period in order to ensure that a stable equilibiiiumcondi-tion is reached and to provide adequate data for subsequent analy`sis of the test. The well should be pumpedA ata constant fate for a period of at least six hours. Preferably, the well is pumped at a constant rate for a period of at least 24 hours'. During this period the bottomhole pressure of the well ,should be measured in order to ensure that the operating pressure is relatively constant. Pressure measurements may be taken at in-tervals or the bottomhole pressure can be measured and recorded continuously during the-period of constant production. In any case, a pressure measurement should be taken before the next succeeding-'step in order to establish a reference point for the pressure build-up data.

After the well is pumped at the desired constant rate and the desired pressure measurements taken, the valve 34 is closed, thus closing the passage extending through packer 32 to annulus 37 to the ow of liquid and eiectively shutting in the well at the face of formation 12. Thus, ow from the formation into the well does not occur during the time the pressure build-,up data is taken. Before or immediately after closing valve 34, the pump should be stopped in order to avoid'vdamage thereto. Preferably, the pump is stopped after closure of the valve in order to ensure that the desired production rate is maintained up to the start of the 'pressure build-up test.

Upon closing valve 34, the bottomhole pressure is recorded as a function of time in order to obtain the desired build-up data. Normally, the pressure sh'ould be recorded continuously during at least the tirst part of the shut-in period when the pressure increases rapidly. Thereafter, the pressure may be recorded at suitable time intervals, e.g., once every minutes as it begins to level out.

The valve 34 should be maintained'closed over such time interval as is necessary to obtain thedesired pressure build-up curve. In some instances, it will. be desired to obtain a complete pressure build-up curve. In this case, valve 34 should remain closed until no further increase in bottomhole pressure is observed. In some cases, it will be unnecessary to carry out the test to lthis conclusion since the nal build-up pressure can be determined accurately by extrapolation of the results of a 4partial build-I up test. In any event, where the well is initially produced at a constant rate as described above,=vfalve 34 should be closed for a period of at least onfetenth of the time during which the well was pumped at" this constant rate. Preferably, the valve is closed for a period of at least one-fourth of this time interval. Thus, if as is preferred, the well is maintained on a constant production rate for a period of 24 hours the valve is kept closed for a period of at least six hours. Thereafter, the valve may be opened and normal operation of the well resumed.

While production of the well at a constant rate prior to closing the valve 34 is preferred, valuable information may be obtained merely by carrying out a pressure buildup test in accordance with the instant invention without rst maintaining a constant production rate. In this case, however, it usually will be desirable to at least monitor the production rate (even though it is not constant) and also measure the bottomhole pressure prior to closing valve 34. These measurements should be taken over a period of at least six hours in order to establish suicient data for a variable rate analysis.

In some instances, of course, time limitations may preclude measuring the production rate and bottomhole pres sure for -any significant interval prior to closing valve 34. In this case, ifthe instantaneous rate can be measured, and this rate mayffbe presumed to have prevailed for some period of time, a pressure build-up test carried out in accordance with the instant invention may provide valun able infomation. Of course, in this instance, a bottomhole pressure measurement should be taken at the time the valve 34 is closed.

Having descgibed certain speciic embodiments of the invention, it will be understood that further modifications may be suggested by those skilled in the art, and it is intended to cover all such modifications as fall within the scope of the appended claimso What is claifned is:

1. In a method of obtaining pressure build-up data. for a well pelnetrating a subterranean formation and adapted for thle recovery of liquids therefrom through an open `prodt'i'ction interval, said well having a production tubing with an inlet therein providing for ilui-d communication between the interior of said tubing and the well annulus exterio'rly of said tubing, anda packer separating said inlet andf said open production interval, the steps comprising:

(a) owing'liquid from said well below said packer through afpassage extending through said packer into the well annulus above said packer;

(b) withdrawing liquid from said annulus into said tubing and producing said liquid through said tubing to the sui-*face of said well;

(c) sensin'g'j'the pressure in the well below said packer;

.(d) thereafter closing said passage to prevent liquid from owing therethrough to cause an increase in the pressureirbelow said packer; and

(e) while s aid passage is closed, sensing the pressure in the well below said packer.

2. The method of claim 1 wherein said well is produced in accordance with steps (a) and (b) at a substantially constant rate'and with a substantially constant level of liquid in saidvannulus.

3. The method of claim 2 wherein said substantially constant product rate is continued for a period of at least six hours before carrying out step (d).

4. The method of claim 2 wherein said substantially constant product rate is continued for a period of at least 24 hours bef-ore the carry out step (d).

5. The method of clam 2 wherein said passage is main tained closed-1in accordance with step (d) for a period of at least one-tenth of the period of said substantially constant production rate.

6. The method of claim 2 wherein said passage is main tained closed in accordance with step (d) for a period of at least one-fourth of the period of said substantially constant production rate.

7. In a Well penetrating a subterranean formation and having an open production interval for the ow of fluid from said formation into said Well, the combination comi prising:

a production tubing in said well having an inlet therein v providing for fluid communication between the interior of "'.said tubing and the well annulus exteriorly of said tubing;

pumping means in said tubing for withdrawing liquid from said annulus and Idisplacing said liquid through said tubing to the surface of said well;

packing means within said well separating said tubing inlet and said open production interval;

means forming a passage extending through said pack ing means and providing for uid communication between said open production interval and said annum lus above said packing means;

valve means in said passage for closing said passage -to uid flow; and

pressure sensing means in said well below said valve means..

8. The combination of claim 7 wherein said pressure sensing means includes means for producing a signal rep resentative of the sensed pressure for transmission of the surface of said well.

References Cited UNITED STATES PATENTS 5/1939 Bowen 73-155 6/1939 ONeill 73-155 2,176,766 10/1939 Johnston 73-155 X 10 RICHARD C.MQUEISSER, Primary Examiner JERRY W.l MYRACLE, Assistant Examiner 

